In plate or sheet rolling by a rolling mill of a plate or a sheet designed so that a pair of top and bottom work rolls are supplied with drive force from independent electric motors, rolling trouble due to warping of the rolled material or flatness defects due to wavy shapes running through the plate/sheet width direction called “waviness”, “full waves”, “small waves”, etc. occur with certainty, so various techniques have been proposed to prevent them.
For example, as art for controlling warping of a rolled material, there is the method of calculating the amount of rolling warping occurring in a pass from the actual values of the rolling load and the rolling torque of the previous pass, calculating a setting changing control amount for the difference in peripheral speeds of the top and bottom rolls to prevent this, and controlling the roll peripheral speeds based on the calculated setting changing control amount for the difference in peripheral speeds of the top and bottom rolls (for example, see Japanese Patent Publication (A) No. 7-164031).
However, the setting changing control amount for the difference in peripheral speeds of the top and bottom rolls for preventing warping changes due to various external factors, so accurately calculating this is difficult. For this reason, while this method exhibits certain effects, it cannot completely eliminate warping.
Further, as art for preventing small waves and waviness, there is the method of controlling the difference in peripheral speeds of the top and bottom rolls so that the rolled material warps upward (for example, see Japanese Patent Publication (A) No. 2002-346617). This is art based on the discovery that wavy shapes extending over the entire plate/sheet width called “small waves” or “waviness” occur at the rolling mill exit side due to the rolled material warping downward and striking the roller table. However, there are also small waves and waviness occurring not due to striking the roller table. In this case, there is no effect.
Further, as one function of control of the drive of electric motors of a rolling mill, control of the load balance to reduce the difference in drive torques between the top and bottom rolls has been realized (for example, see Fuji Electric Journal, Vol. 73, No. 11, pp. 614 to 618 (2000)). This system detects the difference in top and bottom torques to control the difference in top and bottom roll rotational speeds. It focuses on protection of the rolling equipment. To avoid external disturbances in the rolling speed control, the control becomes gentle with a large time constant. No effect of prevention of warping or waviness is obtained.
In this regard, while completely different in object from the present invention, Japanese Patent Publication (A) No. 54-71064 and Japanese Patent Publication (A) No. 60-9509 disclose embodiments similar to the present invention. These inventions are arts deliberately giving a difference to the peripheral speeds or torques of the top and bottom rolls to give the rolled material additional shear plastic deformation, that is, perform so-called differential peripheral speed rolling.